1. Field of the Invention
This invention relates to shaping and cooling glass sheets and particularly to the high speed production of bent glass sheets that are toughened by air quenching, and most particularly, for shaping and heat treating relatively thin glass sheets, particularly those having a nominal thickness of 1/8 inch (3.2 mm) or less. Thinner glass sheets sag more readily than thicker glass sheets at any given elevated temperature above the glass deformation temperature. Hence, it is more difficult to control the shape imparted to thinner glass sheets.
Shaped and tempered glass sheets are widely used as side windows or rear windows in vehicles such as automobiles or the like and, to be suitable for such application, flat glass sheets must be shaped to precisely defined curvatures dictated by the shape and outline of the frames defining the window openings into which the glass side or rear windows are installed. It is also important that the side or rear windows meet stringent optical requirements and that the windows be free of optical defects that would tend to interfere with the clear viewing therethrough in their viewing area.
During fabrication, glass sheets intended for use as shaped windows in vehicles are subjected to thermal treatment to temper the glass for strengthening the same and increasing the resistance of the shaped window to damage resulting from impact. In addition to increasing the resistance of a glass sheet to breakage, tempering also causes a glass sheet to fracture into relatively small, relatively smoothly surfaced fragments that are less injurious than the relatively large, jagged fragments that result from the more frequent breakage of untempered glass.
The commercial production of shaped glass sheets for such purposes commonly includes heating flat sheets to the softening point of the glass, shaping the heated sheets to a desired curvature and then cooling the bent sheets in a controlled manner to a temperature below the annealing range of the glass. During such treatment, a glass sheet is conveyed along a substantially horizontal path that extends through a tunnel-type furnace where the glass sheet is one of a series of sheets that are heated to the deformation temperature of glass and along an extension of said path into a shaping station where each glass sheet in turn is transferred onto a vacuum mold. The vacuum mold lifts and holds the heat-softened glass sheet by suction. At about the same time, a transfer and tempering ring having an outline shape conforming to that desired for the glass sheet slightly inboard of its perimeter moves upstream into a position below the vacuum mold. Release of the vacuum deposits the glass sheet onto the tempering ring. The tempering ring supports the peripheral portion of the glass sheet while it conveys the glass sheet into a cooling station for rapid cooling.
In prior art apparatus, the vacuum mold was either provided with a lower, rigidly curved shaping surface that shaped the heat-softened glass sheet incrementally by suction thereagainst or had a smoothly surfaced flat shaping surface that lifted the flat glass sheet by suction thereagainst and depended on a release of the vacuum within the mold to permit the hot glass sheet to drop by gravity or by a combination of gravity and an additional force onto the tempering ring to develop the shape dictated by the outline configuration of the tempering ring. Such processes as the latter have been called drop forming.
When a rigid, curved surface is adjacent a heat-softened flat glass sheet during the application of suction through said surface, much power is need to obtain the suction necessary to lift and shape a hot glass sheet simultaneously by suction at a rate sufficiently rapid to provide a high speed mass production operation for shaping and tempering glass sheets. When a flat glass sheet is shaped by drop forming, the maximum depth of bend obtainable depends on the glass thickness, glass temperature and distance the glass is dropped. It is difficult to control the shape of thin glass sheets, particularly those heated to excessive temperature. Furthermore, if the drop distance is increased to make possible a deeper bend, more time is needed to lift the glass sheet the longer drop distance, thereby limiting the maximum rate at which bent and tempered glass sheets can be produced.
2. Description of Patents of Interest
U.S. Pat. No. 2,131,873 to Goodwillie shapes one or a pair of glass sheets by sag bending to conform to the upwardly facing concave shaping surface of a solid mold of continuous extent and lowering an upper solid mold of complementary convex shape against the sagged glass sheet within a heating furnace to mechanically press the glass into a more accurate bend. The glass is shown supported throughout its entire surface on the lower mold from the time it sags to conform to the lower mold through the pressing step and during the time the bent glass sheet is removed from the hot atmosphere of the furnace. Such prolonged engagement can produce optical defects in the vision area of the bent glass.
U.S. Pat. No. 2,442,242 to Lewis heats flat glass sheets having a straight leading edge while supported on flat strips until the leading edge engages a straight rib. A pair of heated molds of complementary shape sandwich the hot glass to impress a cylindrical shape thereon in an enclosed bending chamber that forms an extension of a heating chamber. This patent locates the molds in a high temperature atmosphere continuously.
U.S. Pat. No. 2,570,309 to Black sag bends a glass sheet by heating it within a heating furnace while supported on an outline ring-type mold to conform to the mold by gravity sagging and then lifts the gravity sagged sheet on a lower solid pressing mold of concave elevation into pressing engagement against an upper solid pressing mold of complementary shape to complete the spherical bend after conveying the sag-bent glass sheet beyond the heating furnace. The spherically bent sheet is returned to the outline ring-type mold for support with chilling blasts of air to temper the bent sheet.
U.S. Pat. No. 2,663,974 to Robert W. Thomson bends heat softened glass sheets between flexible strips of metal that are weighted and a pair of rigid glass sheet support members that have upper edge surfaces curved convexly in elevation to conform to the shape desired for a rectangular glass sheet after it has been shaped. The weighted strips bear down on the upper surface of the glass sheet to distort the latter to conform to the convexly curved upper edge surfaces. No vacuum or transfer device is included in this apparatus.
U.S. Pat. No. 3,077,753 to August Dammers discloses a press bending mold in which a vertically suspended, heat softened glass sheet is press bent against a rigid, convexly curved, forming dye by pressing a resilient sheet of fabric that is spring loaded to apply pressure against the surface of the glass sheet opposite the surface that faces the rigid die of convex configuration. No transfer to a second shaping member and no vacuum operation is included in this patented construction.
U.S. Pat. No. 3,106,464 to August Dammers moves a rigid die of convex configuration against one surface of a heat softened glass sheet while the opposite surface is forced against a flexible frame to shape the glass sheet and the frame. The frame is then shaped still further at its ends by pistons that engage the opposite ends of the frame against the ends of the convex die. There is no vacuum used in the bending method of this patent.
U.S. Pat. No. 3,265,284 to George F. Ritter Jr. discloses a flexible belt that is located between an upper pressing mold of convex configuration and a lower mold of concave configuration. The lower mold is lifted to bring a heat softened glass sheet carried by the flexible belt into engagement against the downwardly facing surface of the upper mold. When the lower mold retracts, the belt moves the shaped glass sheet onto a roller conveyor that transports the glass through a quenching area where it is cooled. The lack of peripheral support for the glass sheet causes a loss of shape control so that the ultimate shape of the glass sheet after it is cooled can not be controlled from the shape imparted at the press bending station where the glass sheet supported on the flexible belt is sandwiched in pressurized engagement between the upper and lower molds.
U.S. Pat. No. 3,389,984 to Oscar D. Englehart and James S. Shuster is similar to the aforementioned Ritter patent in providing a flexible ribbon of material superimposed over the shaping surface of a pressing mold of concave configuration to provide a run between said concave mold and one surface of a heat softened glass sheet to be press bent. In this patent, the glass sheet is suspended from tongs. The other surface of the glass sheet faces a press bending mold of convex configuration. Relative movement of the molds toward one another presses a shape onto the glass sheet. The glass sheet is then supported solely by tongs for transfer into a cooling area where stresses are imparted to the glass depending upon the rate of cooling. Lack of peripheral support and lack of a vacuum support characterize this press bending operation.
U.S. Pat. No. 3,459,521 to Nedelec supports a flat glass sheet on a hammock while the glass sheet in a heat softened state is pressed between upper and lower molds of complementary configuration. The glass sheet is somehow removed from the hammock and quenched.
U.S. Pat. No. 3,607,187 to Harold A. McMaster lifts a soft, flat glass sheet by lowering a vacuum mold toward said sheet and applying suction through a downwardly facing, permanently curved, shaping surface of a vacuum mold to shape the sheet by suction thereagainst. Much power is needed to provide the suction necessary to shape the entire flat glass sheet to conform to the permanently curved shape of the vacuum mold in incremental portions, particularly those portions most widely spaced from the vacuum mold when other portions are initially engaged by the vacuum mold. This method, if useful at all, is only practical for producing extremely shallow bends and is too time consuming for high speed production. This patent also moves the vacuum mold horizontally over a conveyor belt from a shaping station beyond an enclosed heating furnace to a cooling station. The mold engages the glass sheet by vacuum during its transfer to the cooling station and then releases the vacuum to redeposit the glass sheet without peripheral support onto the conveyor belt. Lack of peripheral support may result in the glass sheet losing its desired shape.
U.S. Pat. No. 3,846,104 to Samuel L. Seymour provides method and apparatus in which glass sheets are conveyed through a furnace on conveyor means, and heated while passing through the furnace to a temperature approaching the glass softening point. At a shaping station beyond the furnace, each glass sheet in turn is lifted by a lower outline shaping mold which raises the glass sheet into engagement with an upper vacuum mold having a shape conforming to that desired for the glass. The upper vacuum mold remains at the shaping station and holds the shaped glass thereagainst as the lower shaping mold retracts to below the level of the conveyor means. A tempering ring shaped to support the bent glass sheet adjacent its marginal or peripheral edge only, moves generally horizontally between the shaping station and a cooling station to receive each shaped glass sheet released by the vacuum mold at the shaping station and transfer it to the cooling station. Therefore, each glass sheet must be lifted in its entirety to an elevated position a minimum vertical distance for transfer to said vacuum mold, a time consuming step.
U.S. Pat. No. 4,092,141 to Robert G. Frank and DeWitt W. Lampman provides similar apparatus with vertically movable sheet transfer means for rapidly removing from the tempering ring each bent glass sheet after its surfaces harden sufficiently to permit it to be conveyed on an additional downstream conveyor. The latter provides a glass sheet supporting surface at an elevation slightly higher than the level at which the glass sheet is supported by the transfer and tempering ring. However, the apparatus of this prior art patent also provides for lifting each glass sheet toward the upper vacuum mold a substantial vertical distance since the vacuum mold remains in an elevated position between successive shaping operations to await the arrival of a subsequent glass sheet at the shaping station.
The invention of U.S. patent application Ser. No. 960,403 of Samuel L. Seymour, filed Nov. 13, 1978, for Glass Sheet Alignment Means and Method of Using now U.S. Pat. No. 4,204,853, discloses a glass sheet shaping and tempering method in which each glass sheet, upon leaving a heating furnace, is elevated by means of a flat vacuum platen which is brought into contact with the upper surface of the heat-softened, flat glass sheet. After the vacuum platen and the glass sheet rise to an elevated position, a shaping and tempering ring is conveyed into a position beneath the glass sheet, the vacuum is released, and the glass sheet drops onto the shaping and tempering ring to effect the bending by the drop forming process. The shaping and tempering ring is then retracted from beneath the vacuum platen and passed into a tempering station where blasts of air are directed onto the opposite surfaces of the drop formed glass sheet to temper the glass. While this arrangement provides a flat surface for the vacuum mold that is easier to smooth than a curved surface, and simplifies change-over from one shape to another since the bending and tempering ring is the only major element of the shaping and tempering apparatus which must be reconstructed or replaced to produce different configurations, drop forming has limitations. For example, the depth of bend that can be accomplished thereby without losing control over the overall shape of the treated glass sheet is limited.
Prior to the present invention, the glass sheet bending art lacked a glass sheet shaping and tempering technique that comprised a vacuum mold having a glass engaging surface as smooth as that of flat vacuum molds of the prior art, and that also shaped the glass sheet to a shape approximately its final desired shape within a heating furnace before releasing the glass sheet onto a shaping and tempering ring to make it possible to increase the speed of a mass production operation for shaping and tempering glass sheets, particularly those thinner than 3.2 mm nominal thickness. Shaping thin glass sheets within a heating furnace saves energy because it avoids the need to overheat the glass, which cools rapidly en route to a shaping station outside the furnace.
However, it is difficult to control the shape and temperature of vacuum molds permanently installed within a furnace. It is also difficult to obtain access to repair or provide routine maintenance for a mold that is permanently installed within a furnace.